This invention relates to methods and apparatus for analyzing failures in semiconductor circuits.
LIVA (Light Induced Voltage Alterations), U.S. Pat. No. 5,430,305, and TIVA (Thermally Induced Voltage Alterations), U.S. Pat. No. 6,078,183, have demonstrated significant capability to for fault isolation in semiconductor circuits. A difficulty with both techniques is their use of a constant current bias, whereas integrated circuits operate on constant voltage bias. OBIRCH (Optical Beam Induced Resistance Changes), U.S. Pat. No. 5,952,837, utilizes constant voltage bias, however, tests have shown that LIVA/TIVA is as much as two orders of magnitude more sensitive (xe2x80x9cBackside Localization of Open and Shorted Interconnectionsxe2x80x9d E. I. Cole, Jr. et al., International Reliability Physics Symposium, pp. 129-136, 1998).
The current invention utilizes a new technique for fault isolation in semiconductors that makes use of the constant current sensing of LIVA/TIVA, while allowing for use of constant voltage bias on the integrated circuit. The operational advantages include: correct biasing of the integrated circuit, ability to change the state of the integrated circuit without also having to change the current bias conditions, and ability to utilize the technique on lines other than power lines. In addition, the technique is significantly more sensitive (at least one order of magnitude) than the standard LIVA/TIVA approach.
LIVA, TIVA, and OBIRCH all utilize a scanned optical beam from a laser scanning microscope to stimulate a device under test (DUT). Nominally, the DUT consists of a semiconductor integrated circuit, which consists of passive (resistors, capacitors) and active (transistors) elements on a semiconductor substrate. Modern integrated circuits can also contain electro-mechanical and sensor elements. In the case of LIVA, the stimulation is due to carrier production by the optical beam in the semiconductor. For TIVA and OBIRCH, the stimulation is thermal only (e.g. the wavelength of the laser is selected such that no carrier production occurs). In each case, the response of the device under test is recorded versus scanner position to produce an image. Nominally, the recording and display utilize an analog to digital (A/D) conversion, a general-purpose computer and a monitor. The response image corresponds to the reflected light image that can also be recorded by the microscope. The response image can indicate the presence and character of a number of failures as described in the referenced patents and papers.
The distinction between the LIVA/TIVA and OBIRCH techniques lies in the use of a constant current source versus constant voltage source for the DUT bias. The constant voltage approach is consistent with normal operation of an integrated circuit; however, the constant current approach has several orders of magnitude more sensitivity. The current invention seeks to maintain the optimum bias approach (constant voltage) while producing signal strengths associated with the constant current approach. This desire can be realized by recognizing that the DUT bias requirement lies in low frequency or DC regime, where as the signals lie at higher, AC, frequencies and can thereby be separated by suitable circuitry. An additional benefit is to separately optimize the bias circuitry and the signal circuitry, which produces increased signal-to-noise.
One method for performing this separation applies constant voltage bias to the DUT through a choke coil. The coil does not affect the DC bias of the DUT. However, when the DUT is stimulated by the scanned optical beam it momentarily attempts to draw more (or less) current from the bias circuit. The choke coil temporarily suppresses this change in current, i.e. it acts as an AC constant current source. The voltage across the DUT (or coil) will change just as for the LIVA/TIVA systems. An amplifier can be used to increase the signal strength.
Additionally, optimization of the bias circuitry and the sensing circuitry is now separated. In both LIVA/TIVA and OBIRCH, the bias circuits (power supplies) are producing the voltage or current (respectively) signal that is then amplified. Power supplies are not known to be optimized for their signal generation properties.
Clearly other circuits (more complex, with feedback, etc.) can also be utilized to obtain the same separation of circuit bias and signal sensing, however, the basic operational principles of separation in the frequency domain would be identical. Scanned electron beams and ion beams are also utilized for stimulation of semiconductor circuits in order to locate failures. The invented sensor arrangement can also be utilized with these forms of stimulation of the circuit.
The present invention provides an apparatus for integrated circuit analysis. The apparatus includes an irradiation component configured to irradiate an integrated circuit, a constant voltage source, one or more current chokes placed between the constant voltage source and one or more connections to the integrated circuit, and one or more voltage amplifiers. The irradiation component introduces changes in the electrical state of the integrated circuit. The one or more voltage amplifiers produces a state signal relating to the changes introduced by the irradiation component in the integrated circuit. The current choke separates the function, a DC function, of supplying a voltage bias to the integrated circuit from the function, an AC function, of signal generation. This allows separate optimization of the two functionsxe2x80x94the ability to correctly bias the high signal-to-noise ratios for the resulting signal. This optimization improves the sensing abilities of the circuit.
In accordance with further aspects of the invention, the apparatus further includes a component for focusing and scanning the irradiation over the integrated circuit, wherein the scanning acts to modulate the irradiation.
In accordance with other aspects of the invention, the irradiation is from one of a laser source, an electron beam or an ion beam.
In accordance with still further aspects of the invention, the apparatus further includes a component for recording and displaying an image of the state signal as a function of scan position.
In accordance with yet other aspects of the invention, the apparatus further includes a component for producing an image of the integrated circuit. The produced image is utilized to correlate the state signal to corresponding positions on the integrated circuit.
As will be readily appreciated from the foregoing summary, the invention provides an integrated circuit analysis device with improved sensing capabilities.